A. Field of the Invention
The invention relates to measuring variations in a film.
B. Background
During fabrication of microelectronic devices, thin films of metals and metal alloys are deposited on silicon wafers for use as electrical conductors, adhesion-promoting layers, and diffusion barriers. Thickness variations in the metal films can modify their electrical and mechanical properties, such as resistance-capacitance delay and photolithography resolution, thereby affecting the performance of the devices in which they are used. In electro-chemically deposited (ECD) copper films, for example, thickness variations are commonplace.
After these films are deposited, they can be subject to processes such as chemical-mechanical polishing (CMP) that modify their thickness and uniformity. In particular, uniformity is an important property that must be closely controlled in order to assure uniform behavior of the individual microelectronic devices. Generally, film thickness at any point on the film must be within a few percent of the mean film thickness to be considered uniform enough for use in semiconductor manufacturing. Because of these rigid tolerances, thickness and uniformity measurements are often used as quality-control parameters during and/or after the microprocessor's fabrication.
On method of generating thickness and uniformity measurements is called Impulsive Stimulated Thermal Scattering (ISTS), which is described, for example, in pending and issued U.S. Pat. No. 5,633,711 (entitled MEASUREMENT OF MATERIAL PROPERTIES WITH OPTICALLY INDUCED PHONONS); U.S. Pat. No. 5,546,811 (entitled OPTICAL MEASUREMENT OF STRESS IN THIN FILM SAMPLES); and U.S. Pat. No. 5,812,261 (entitled METHOD AND DEVICE FOR MEASURING THE THICKNESS OF OPAQUE AND TRANSPARENT FILMS), the contents of which are incorporated herein by reference. In ISTS, a first laser pulse initiates a sound wave that propagates in a plane of the film. A second laser pulse measures a frequency of the sound wave. The frequency of the sound wave relates to film thickness.
Another thickness measurement device is the four point probe, for instance the products sold under the trade name PROMETRICS - RS by KLA Tencor.
Typically film measurements are made along a line spanning a diameter or edge of the film, a concentric circular pattern over the entire surface of the film, or at random points on the surface of the film.
In most semiconductor manufacturing processes, time is at a premium. The less time spent on each device, the more devices can be produced with a particular item of equipment. The more devices can be produced with that item, the lower the costs of production.
Traditionally, measurement of the thickness of films in microelectronic devices has been considerably slower than the production of the films. Accordingly, manufacturers have elected to measure only some of the films produced, i.e. a type of spot checking. For instance, with respect to copper films, manufacturers have typically measured only one out of every 13 films. This results in considerable uncertainty as to the quality of the other films.